Nowadays international regulations and laws of individual countries require increasingly high levels of energy efficiency increasingly for electric motors.
Among the several types of electric motors, the highest yields are obtained with permanent-magnet motors.
Usually permanent-magnet electric motors, synchronous or brushless, comprise a rotor, in which are arranged the permanent magnets, a stator, in the slots of which a three-phase winding is accommodated, and an electronic converter.
Thanks to this technology it is possible to obtain levels of yield of the motor which fall under class IE4 of the IEC standard, which is currently the highest class.
In the past, permanent-magnet motors were widely used in industrial drives which require high levels of dynamic performance and reduced space occupation. Today, however, their use is also spreading to sectors that are currently dominated by the asynchronous motor, such as the sector of pumps and fans.
However, the noise and the mechanical vibrations that they generate are non-negligible aspects in residential and commercial contexts, where acoustic comfort is an essential requirement.
Another non-negligible drawback of this type of motor is their relatively high cost. This is due to the fact that, presently, the permanent magnets used are made with alloys based on rare earth elements, such as neodymium, dysprosium and samarium.
These alloys are characterized by excellent residual induction and by considerable intrinsic coercive force, characteristics which ensure a high flow and a considerable resistance to demagnetization, with a relatively low volume of material, giving the motor high performance levels with reduced space occupancy.
However, their use is discouraged by the high price, which is unstable over time, and in recent years has increased significantly.
Conventionally, rare earth elements can be substituted with ferrite, the price of which markedly lower and more stable over time, given its high availability.
However, nowadays, with ferrite it is not possible to obtain the same levels of performance that can be obtained with rare earth elements. In fact, the maximum values of residual induction and intrinsic coercive force that can be obtained with ferrite are substantially equal to one third of the values that can be obtained with magnets that use rare earth elements. In order to achieve the same performance levels it is necessary to accommodate a greater volume of magnet.
In the most common configuration of permanent-magnet motors, the magnets are glued onto the surface of the rotor according to the SPM (“Surface-mounted Permanent Magnet”) configuration.
Another possible configuration is one in which the magnets are accommodated inside the rotor in adapted slots, according to the IPM (“Interior Permanent Magnet”) configuration. In this case the magnets can be arranged radially in the rotor, with the direction of magnetization being perpendicular to the radius.
IPM configurations make it possible to accommodate a greater volume of magnet than SPM configurations, thus favoring the use of ferrite.
With IPM motors it is therefore possible to contain costs; however, in order to fully exploit the potential in terms of motor torque, it is often necessary to equip the microprocessor of the electronic converter with a different control algorithm to the one usually used for SPM configurations. Such algorithm is in general more complex as well, since it has to take into account the variation of inductance levels with the current, and it should furthermore be calibrated to the motor: this has made standardization of the algorithms difficult owing to the great many possible types of IPM and as a consequence it has impeded the large-scale commercial spread of IPM drives.
Furthermore, the IPM rotor configuration has, usually with respect to the SPM solution, a higher moment of inertia, a higher inductance, more uneven torque as a function of time, and a nonlinear torque-current progression.
For the above mentioned reasons, the rotor configuration of the SPM type is often preferred to the IPM configuration in industrial automation applications.
In residential and commercial contexts on the other hand it is often the noise of the IPM configuration, which tends to be more pronounced than an SPM configuration, that discourages its use. Noise and mechanical vibrations of an electric motor are aspects that should not be underestimated, especially in residential and commercial contexts, where acoustic comfort is essential.
Furthermore, in order to facilitate the penetration of a different type of electric motor in market sectors that nowadays are occupied by other types of electric motors, it is necessary that it be offered at a competitive price.